8/22/2019 Carbo Test

1/8

Iodine/Potassium Iodide Test

Shows positive test for:Starch

Reactions:Iodine complexes with starch to form a blue-black product. Note that other

polysaccharides may give other colors including blue or red.

How to perform the test:Two ml of a sample solution is placed in a test tube. Two drops of

iodine/potassium iodide solution and one ml of water are added.

A positive test is indicated by:The formation of a blue-black complex.

a negative test (left) and a positive test (right)

8/22/2019 Carbo Test

2/8

The Molisch Test

Shows positive test for:All carbohydrates. Monosaccharides give a rapid positive test. Disaccharidesand polysaccharides react slower.

Reactions:The test reagent dehydrates pentoses to form furfural (top reaction) and

dehydrates hexoses to form 5-hydroxymethyl furfural (bottom reaction). The

furfurals further react with -naphthol present in the test reagent to produce

a purple product (reaction not shown).

How to perform the test:

8/22/2019 Carbo Test

3/8

Two ml of a sample solution is placed in a test tube. Two drops of the Molisch

reagent (a solution of -napthol in 95% ethanol) is added. The solution is

then poured slowly into a tube containing two ml of concentrated sulfuric acidso that two layers form.

A positive test is indicated by:The formation of a purple product at the interface of the two layers.

a negative test (left) and a positive test (right)

Benedict's Testhttp://www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/

Shows positive test for:Reducing sugars

Reactions:Reducing sugars are oxidized by the copper ion in solution to form a carboxylicacid and a reddish precipitate of copper (I) oxide.

http://www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/http://www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/http://www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/

8/22/2019 Carbo Test

4/8

How to perform the test:One ml of a sample solution is placed in a test tube. Two ml of Benedict'sreagent (a solution of sodium citrate and sodium carbonate mixed with asolution of copper sulfate) is added. The solution is then heated in a boiling

water bath for three minutes.

A positive test is indicated by:The formation of a reddish precipitate within three minutes.

a negative test (left) and a positive test (right)

Barfoed's Test

Shows positive test for:Reducing monosaccharides

Reactions:Reducing monosaccharides are oxidized by the copper ion in solution to form acarboxylic acid and a reddish precipitate of copper (I) oxide within threeminutes. Reducing disaccharides undergo the same reaction, but do so at aslower rate.

8/22/2019 Carbo Test

5/8

How to perform the test:One ml of a sample solution is placed in a test tube. Three ml of Barfoed'sreagent (a solution of cupric acetate and acetic acid) is added. The solution isthen heated in a boiling water bath for three minutes.

A positive test is indicated by:The formation of a reddish precipitate within three minutes.

a negative test (left) and a positive test (right)

Seliwanoff's Test

Shows positive test for:Ketoses

8/22/2019 Carbo Test

6/8

Reactions:The test reagent dehydrates ketohexoses to form 5-hydroxymethylfurfural. 5-hydroxymethylfurfural further reacts with resorcinol present in the test reagentto produce a red product within two minutes (reaction not shown). Aldohexoses

react to form the same product, but do so more slowly.

How to perform the test:One half ml of a sample solution is placed in a test tube. Two ml ofSeliwanoff's reagent (a solution of resorcinol and HCl) is added. The solution isthen heated in a boiling water bath for two minutes.

A positive test is indicated by:The formation of a red product.

a negative test (left) and a positive test (right)

8/22/2019 Carbo Test

7/8

Bial's Test

Shows positive test for:Pentoses

Reactions:The test reagent dehydrates pentoses to form furfural. Furfural further reactswith orcinol and the iron ion present in the test reagent to produce a bluish

product (reaction not shown).

How to perform the test:Two ml of a sample solution is placed in a test tube. Two ml of Bial's reagent(a solution of orcinol, HCl and ferric chloride) is added. The solution is thenheated gently in a Bunsen Burner or hot water bath. If the color is not obvious,more water can be added to the tube.

A positive test is indicated by:The formation of a bluish product. All other colors indicate a negative result for

pentoses. Note that hexoses generally react to form green, red, or brownproducts.

8/22/2019 Carbo Test

8/8

two negative tests (left, middle) and a positive test (right)